1. Field of the Invention
This invention relates to a humidity sensor composed of a sintered compact of a metal oxide, and adapted to detect relative humidity as a change in electrical resistance.
2. Description of the Prior Art
A humidity sensor has a sensing portion which usually comprises a humidity-sensitive resistor composed of an organic polymer, such as a polyamide resin, polyvinyl chloride or polyethylene, or a metal oxide, such as Fe.sub.2 O.sub.3 Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, or V.sub.2 O.sub.5. A metal oxide is considered higher in chemical and physical stability, and more promising as a sensor material than an organic polymer.
Conventional metal oxides are, however, defective in a number of respects. For example, they are high in specific resistance, and have a low humidity dependence of resistance. A hysteresis exists in their humidity-resistance characteristics when they absorb and desorb moisture. Moreover, they fail to remain stable in a highly humid atmosphere for a long period of time. They are, therefore, not suitable as a material for humidity sensors.